Vapor phase growth method

ABSTRACT

It is to provide a vapor phase growth method in which an epitaxial layer consisting of a compound semiconductor such as InAlAs, can be grown, with superior reproducibility, on a semiconductor substrate such as Fe-doped InP. In vapor phase growth method for growing an epitaxial layer on a semiconductor substrate, a resistivity of the semiconductor substrate at a room temperature is previously measured, a set temperature of the substrate is controlled depending on the resistivity at the room temperature such that a surface temperature of the substrate is a desired temperature regardless of the resistivity of the semiconductor substrate, and the epitaxial layer is grown.

DESCRIPTION

1. Technical Field

The present invention relates to a vapor phase growth method for growingan epitaxial layer on a semiconductor substrate. In particular, thepresent invention relates to a technique for improving a characteristicand a surface morphology of an epitaxial layer.

2. Background Art

Conventionally, semiconductor elements have been widely used, includingthose provided by growing, on an InP substrate, an epitaxial layerconsisting of a compound semiconductor (e.g., InGaAs layer, AlGaAslayer, InAlAs layer, AlInGaAs layer, InGaAsP layer) by the metalorganicchemical vapor deposition (MOCVD) or the molecular beam epitaxy (MBE)for example.

However, when the conventional technique is used to grow an epitaxiallayer consisting of a compound semiconductor, e.g., InAlAs, on an InPsubstrate, there may be a case where a surface of the epitaxial layerhas an abnormal morphology. This abnormal morphology is one of causingfactors of the deterioration of the element characteristic of asemiconductor element. Thus, an improvement of the morphology of thesurface of the epitaxial layer is an important problem to be solved.

For example, the present inventors have proposed a vapor phase growthmethod by which, in a process for sequentially epitaxially growing anInGaAs layer or an InGaAsP layer and an InP layer on an InP substrate,an abnormal morphology called a crosshatch can be effectively preventedfrom being generated at the surface of the InP layer (Patent Publication1). Specifically, a semiconductor wafer having a warpage at the backface of 20 μm or less is used as a substrate to reduce a space betweenthe back face of the semiconductor substrate and a substrate supporttool to suppress raw material gas from going to the back face of thesubstrate, thereby preventing an abnormal morphology from being causedat the surface of the epitaxial layer.

-   Patent Publication 1: Japanese Patent Unexamined Publication No.    2003-218033

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, it was found that, even when the above technique according tothe prior application was used to grow an epitaxial layer consisting ofa compound semiconductor, e.g. InGaAs layer, AlGaAs layer, InAlAs layer,AlInGaAs layer, on an InP substrate, some substrate causes the epitaxiallayer grown on the substrate to have an abnormal characteristic orsurface morphology.

The present invention has been made to solve the above problem. Anobject of the present invention is to provide a vapor phase growthmethod by which, on a semiconductor substrate such as Fe-doped InP, anepitaxial layer consisting of a compound semiconductor, e.g., InAlAs,can be grown with superior reproducibility.

Means for Solving the Problem

Hereinafter, how the present invention is achieved will be brieflydescribed.

First, the present inventors used the molecular beam epitaxy(hereinafter simply referred to as MBE) to grow an InAlAs layer onvarious types of Fe-doped InP substrates. Specifically, a plurality ofInP wafers were cut out of an Fe-doped InP single crystal manufacturedby the Liquid Encapsulated Czochralski and the cut member was used as asubstrate. Here a distance (mm) from a position just below a shoulderpart of the resultant InP single crystal (body part starting position)is the cutting position of InP wafer. The result showed that somesubstrate caused the grown epitaxial layer to have an abnormal surfacemorphology. In other words, it was found that even when substrates cutout of the same InP single crystal are used, the resultant grownepitaxial layers change depending on the cutting position thereof.

Next, in order to investigate a cause of this, Fe-doped InP substrateshaving different cutting positions were introduced into a single MBEapparatus to subsequently heat the substrates with a single settemperature to directly measure the surface temperatures of thesubstrates by a pyrometer. The result is shown in FIG. 1. FIG. 1illustrates a relation between the cutting positions of the substratesfrom an Fe-doped InP single crystal and surface temperatures of thesubstrates. In FIG. 1, the □ mark represents a case where a settemperature was 550° C. while the ◯ mark represents a case where a settemperature was 600° C.

As can be seen from FIG. 1, regardless of the single set temperature,the surface temperature dramatically changes depending on a cuttingposition of the substrate. When a substrate cut out from an upper partof the InP single crystal (cutting position: 0 to 10 mm) is used andwhen a substrate cut out from the lower part (cutting position: 100 to120 mm) is used in particular, a difference of 20° C. or more was causedin the surface temperature of the substrates. When a substrate having acutting position of 100 to 120 mm is used, the surface temperature ofthe substrate was 20 to 30° C. higher than the set temperature.

In the above-described experiment, the surface temperature of thesubstrate was higher than the set temperature. However, some MBEapparatus may cause the surface temperature of the substrate to be lowerthan the set temperature. However, this case is also involved with aphenomenon in which the surface temperature changes depending on thecutting position of the substrate.

Next, with regards to substrates cut out from a single Fe-doped InPsingle crystal, the resistivity and Fe concentration were measured toinvestigate a relation with the cutting position. FIG. 2 shows arelation between the resistivity and the cutting position. FIG. 3 showsa relation between the Fe concentration and the cutting position. As canbe seen from FIGS. 2 and 3, the resistivity and the Fe concentrationboth change depending on the cutting position from the Fe-doped InPsingle crystal. Specifically, as shown in FIG. 2, the resistivitygradually increases with an increase of the cutting position and isalmost constant when the cutting position is 100 mm or more. As shown inFIG. 3, the Fe concentration gradually increases with an increase of thecutting position and remarkably increases when the cutting position is100 mm or more.

When these measurement results are compared with FIG. 1, the change tothe cutting position is almost the same as those of FIG. 1 and FIG. 2.Thus, it can be said that the surface temperature of the substrate has acorrelation with a resistivity of the substrate, not with the Feconcentration. This is presumably caused because, when the substrate isheated in vacuum as in the MBE method, an influence by the radiation isdominant and thus the substrate resistivity has an influence on thesurface temperature of the substrate.

Thus, the present invention was achieved by finding that, based on therelation between the substrate resistivity and the surface temperature,an actual surface temperature of the substrate can be a desiredtemperature by adjusting a set temperature depending on apreviously-measured substrate resistivity, thus stabilizing the qualityof an epitaxial layer to be grown.

That is, according to the present invention, a vapor phase growth methodfor growing an epitaxial layer on a semiconductor substrate, comprises:previously measuring a resistivity of the semiconductor substrate at aroom temperature; controlling a set temperature of the substratedepending on the resistivity at the room temperature such that a surfacetemperature of the substrate is a desired temperature regardless of theresistivity of the semiconductor substrate; and growing the epitaxiallayer. Furthermore, the surface temperature of the substrate changesdepending on the thickness of the substrate or a heating method. Thus,if a relation between a set temperature for a resistivity of thesemiconductor substrate and an actual surface temperature of thesubstrate with regards to each of them is found, a temperature forallowing the surface temperature of the substrate to be a desiredtemperature can be set easily.

Furthermore, the semiconductor substrate can be a compound semiconductorsuch as InP or Fe-doped InP. When an InP substrate or an Fe-doped InPsubstrate is used, an epitaxial layer to be grown may be the one thatmay have a favorable lattice matching with InP, such as InGaAs, AlGaAs,InAlAs, AlInGaAs, InGaAsP.

The above-described vapor phase growth may use the molecular beamepitaxy.

Effect of the Invention

According to the present invention, in a process for subjecting anepitaxial layer consisting of a compound semiconductor, e.g., InAlAslayer, to a vapor phase growth on a semiconductor substrate, e.g.,Fe-doped InP, the change of the substrate temperature due to thesubstrate resistivity is considered and the substrate temperature isappropriately set to fix the substrate temperature at a desiredtemperature. This provides an effect in which the epitaxial layer havinga stable quality can be grown with superior reproducibility and asemiconductor element having superior characteristic can be manufacturedstably.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] This is a graph illustrating a relation between a cuttingposition from an InP single crystal and a surface temperature of asubstrate.

[FIG. 2] This is a graph illustrating a relation between a cuttingposition from an InP single crystal and a resistivity of the substrate.

[FIG. 3] This is a graph illustrating a relation between a cuttingposition from an InP single crystal and an Fe concentration of thesubstrate.

[FIG. 4] This illustrates a temperature profile in a vapor phase growthin an embodiment.

[FIG. 5] This is a graph illustrating a temperature dependency of aresistivity of an InAlAs layer.

[FIG. 6] This is a graph illustrating a temperature dependency of a Sidoping efficiency of an InAlAs layer.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the drawings.

First, Liquid Encapsulated Czochralski (LEC) was used to grow anFe-doped InP single crystal material in a direction of (100). Then, thisFe-doped InP single crystal was processed to have a cylindrical shapehaving a diameter of 2 inches. Then, Fe-doped InP wafers having athickness of 350 μm were cut out.

Then, on these substrates, undoped InAlAs layers were grown by the MBEmethod. FIG. 4 illustrates the temperature profile until the growthaccording to this embodiment is reached. As shown in FIG. 4, a substratewas subjected to a thermal cleaning processing with 550° C. before anInAlAs layer is grown. Thereafter, an undoped InAlAs layer wasepitaxially grown on the substrate to have a thickness of 1 μm with aset temperature of 500° C. The thermal cleaning processing was performedfor 5 minutes and the undoped InAlAs layer was grown for 60 minutes.

In this embodiment, various Fe-doped InP substrates having differentcutting positions were measured with regards to the substrateresistivities at a room temperature. Based on the substrateresistivities, a set temperature was adjusted so that an actualsubstrate temperature was fixed at a desired temperature to perform athermal cleaning processing and the growth of the undoped InAlAs layer.Specifically, when an Fe-doped InP substrate having a substrateresistivity at a room temperature of about 1×10⁸Ω·cm was used, a settemperature of the substrate for a thermal cleaning processing wasdetermined as 530° C. and a set temperature of the substrate for thegrowth of the undoped InAlAs layer was determined as 480° C., therebycontrolling the substrate temperature to be a desired temperature.

It is noted that, this temperature setting is effective in thisembodiment and a set temperature may be different depending on a factorsuch as an MBE apparatus to be used or a thickness of a substrate. Inother words, a surface temperature of a substrate changes depending on athickness of the substrate or a heating method. Thus, if a relationbetween a set temperature to a resistivity of a semiconductor substrateat a room temperature and an actual surface temperature of the substrateis found, a set temperature for allowing the surface temperature of thesubstrate to be a desired temperature can be determined easily. Forexample, set temperatures in this embodiment are respectively set 20° C.lower in order to allow the substrate temperature to be a desiredtemperature (550° C. or 500° C.). However, a set temperature also may beset, contrary to the above case, to be higher than a desired temperaturedepending on an MBE apparatus to be used.

On the other hand, a plurality of Fe-doped InP substrates having thesame resistivity as the above-described one were used for comparison toperform a thermal cleaning processing and a growth of an undoped InAlAslayer under conditions in which a set temperature of the substrate wasfixed (550° C. at the thermal cleaning processing and 500° C. at thegrowth of the undoped InAlAs).

With regards to the semiconductor element obtained by theabove-described method, the surface morphology of the undoped InAlAslayer was observed. The result showed that, when the set temperature wasadjusted depending on the resistivity at a room temperature and anactual surface temperature of the substrate during the thermal cleaningprocessing was retained at 550° C., the surface of the undoped InAlAslayer was not rough and a favorable epitaxial layer could be grown.

When the set temperature was fixed at 550° C. on the other hand, thesurface of the undoped InAlAs layer was rough. This is presumably causedbecause the surface temperature of the substrate was much higher thanthe set temperature (550 ° C.) during the thermal cleaning processingand thus the substrate surface became rough. When a substrate having aresistivity at a room temperature of 1×10⁸Ω·cm was used in particular, adifference between the set temperature and an actual surface temperatureof the substrate was increased (see FIGS. 1 and 2). Thus, theabove-described phenomenon clearly appeared.

The obtained semiconductor elements were measured with regards to theresistivity of the undoped InAlAs layer. The result showed that, when asubstrate having a resistivity at a room temperature of 1×10⁸Ω·cm wasused and the set temperature was adjusted depending on the resistivityand an actual surface temperature of the substrate during the growth ofthe undoped InAlAs layer was maintained at 500° C., resistivities of theundoped InAlAs layer were all equal to or higher than 1×10⁶Ω·cm, thusrealizing a high resistivity.

When the set temperature was fixed on the other hand, the resistivity ofthe undoped InAlAs layer, which was equal to or higher than 1×10⁶Ω·cm inthe above embodiment, lowered to 5×10⁴Ω·cm. This was presumably causeddue to a temperature dependency of the resistivity of the undoped InAlAslayer.

For example, an experiment by the present inventors showed that theresistivity of an InAlAs layer showed the temperature dependency asshown in FIG. 5. As can be seen from FIG. 5, the resistivity of theInAlAs layer remarkably lowers when the substrate temperature during thegrowth is 520° C. or more. In other words, in the comparison example,regardless of the set temperature of the substrate during the growth ofthe undoped InAlAs layer of 500° C., an actual substrate temperatureincreased to a value equal to or higher than 520 ° C. It is noted thatthe graph shown in FIG. 5 is the one regarding an InAlAs layer grown bygrowth conditions different from those of this embodiment. Thus, anabsolute value of the resistivity of the InAlAs layer is not alwaysequal to that of this embodiment.

Next, the same Fe-doped InP substrates as the above-described substratewere used and Si-doped InAlAs layers were grown on these substrate bythe MBE method. During the growth, Si was doped in an amount throughwhich a doping concentration of 2×10¹⁹cm⁻³ was obtained and the growthconditions were the same as the above-described conditions for theundoped InAlAs. For comparison, a plurality of Fe-doped InP substrateshaving the same resistivity of the above one were used and a thermalcleaning processing and the growth of an Si-doped InAlAs layer wereperformed under conditions in which a set temperature of the substratewas fixed.

With regards to the obtained semiconductor element, an Si dopingconcentration (carrier concentration) of the Si-doped InAlAs layer wasmeasured. The result showed that, when substrates having a resistivityat a room temperature equal to or higher than 1×10⁸Ω·cm were used and aset temperature was adjusted depending on the resistivity and an actualsurface temperature of the substrates during the growth of the InAlAslayer was maintained at 500° C., all Si dope concentrations of the Sidope InAlAs layers were 2×10¹⁹cm⁻³.

When the set temperature was fixed on the other hand, the Si dopingconcentration lowered to 1×10¹⁹cm⁻³. This is presumably caused by thetemperature dependency of the Si doping efficiency of the Si-dopedInAlAs layer.

For example, an experiment by the present inventors shows that the Sidoping efficiency of the InAlAs layer shows a temperature dependency asshown in FIG. 6. As can be seen from FIG. 6, the Si doping concentrationof the InAlAs layer lowers when the substrate temperature during thegrowth is 500 ° C. or more. In other words, in spite of the settemperature of the substrate during the growth of the Si-doped InAlAslayer of 500° C. in the comparison example, an actual substratetemperature presumably increased to a value of 500° C. or more.

As described above, in a process for subjecting an epitaxial layerconsisting of an undoped InAlAs or an Si-doped InAlAs to a vapor phasegrowth on an Fe-doped InP substrate, a resistivity of the semiconductorsubstrate at a room temperature is previously measured to control theset temperature of the substrate depending on the resistivity of thesemiconductor substrate. As a result, a surface morphology, resistivityand doping concentration of a grown epitaxial layer could be improvedand thus an epitaxial layer having a stable quality could be grown witha superior reproducibility.

As described above, the invention made by the present inventors has beenspecifically described based on embodiments. However, the presentinvention is not limited to the above embodiments and can be changed ina range not departing from the gist thereof.

For example, although this embodiment has described an example in whichthe MBE method was used to grow an InAlAs layer on an Fe-doped InPsubstrate, a growth method for growing an epitaxial layer such that asurface temperature changing depending on the resistivity is fixed canprovide the same effect regardless of the type of the grown epitaxiallayer. The present invention is not limited to a substrate or a growthmethod to be used, as can be seen from the above-described description.

1. A vapor phase growth method for growing an epitaxial layer on asemiconductor substrate, comprising: previously measuring a resistivityof the semiconductor substrate at a room temperature; controlling a settemperature of the substrate depending on the resistivity at the roomtemperature such that a surface temperature of the substrate is adesired temperature regardless of the resistivity of the semiconductorsubstrate; and growing the epitaxial layer.
 2. The vapor phase growthmethod as claimed in claim 1, wherein the semiconductor substrate is acompound semiconductor.
 3. The vapor phase growth method as claimed inclaim 2, wherein the semiconductor substrate is an InP substrate.
 4. Thevapor phase growth method as claimed in claim 3, wherein thesemiconductor substrate is an Fe-doped InP substrate.
 5. The vapor phasegrowth method as claimed in any of claims 1 to 4, wherein a molecularbeam epitaxy is used to grow an epitaxial layer.